| Summary: | Diet additives that influence immunity and disease resistance of aquatic species continue to receive considerable attention due their potential to positively influence intensive aquaculture. Prebiotics, probiotics and essential oils are some of the prominent types of feed additives that may influence the microbiota of the gastrointestinal tract (GIT). It has been documented that microbiota of the GIT play important roles in affecting the nutrition and health of the host organism. Thus, various means of altering the intestinal microbiota to achieve favorable effects such as enhancing growth, digestion, immunity and disease resistance of the host organism continue to be pursued.The first line of defense within the GIT is the mucosa that separates the gut microbiota from direct contact with the epithelial cells of the GIT. It is because of this direct contact with the mucus that the immune system of the GIT, often referred to as gut-associated lymphoid tissue or GALT, has developed mechanisms to distinguish between potentially pathogenic bacteria and the normal, commensal bacteria. In the event that potentially pathogen bacteria are detected, the cellular and humoral mechanisms of the GALT activate the innate immune system and subsequently the adaptive immune system to prevent bacteria from causing and/or spreading infection. It is anticipated that continued advancements with diet additives that influence the GIT will allow nutritional modulation of immunity to be increasingly used as an effective and relatively inexpensive means of combating diseases in aquaculture.
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